<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      const arr0 = ['css', 'html'];
      const arr1 = [100, 2, 3];
      const arr2 = ['a', ...arr1, 'b', ...arr0, 'c', ...arr0];
      const arr3 = [...arr2, 'a', 'b'];
      console.log(arr2);
      console.log(arr3);
      // console.log(arr2.concat(arr1));
    </script>
  </body>
</html>
